Polyphase converter circuits are used today in a wealth of power electronics applications. In this case, the requirements for such a converter circuit are firstly to produce harmonics as little as possible in an electrical AC voltage system which is generally connected on the input side to the converter circuit and also on phases of an electrical load which is more conventionally connected to the converter circuit, and secondly to transmit powers which are as high as possible using as few electronic components as possible. A suitable polyphase converter circuit is specified in EP 0913 918 A2 and shown in FIG. 1. Therein, the converter circuit is in the form of an 18-pulse, three-phase converter circuit and has one converter circuit element for each phase, each converter circuit element comprising an 18-pulse rectifier unit, a DC voltage circuit which is connected to the rectifier unit and an inverter unit which is connected to the DC voltage circuit. In accordance with EP 0913 918 A2 or in accordance with FIG. 1, a first AC voltage output of each inverter unit forms a phase connection. Second AC voltage outputs of the inverter units of the converter circuit are also star-connected. Furthermore, the converter circuit has a single transformer having a primary winding which is generally connected to the electrical AC voltage system. Owing to the pulse number of 18 for the converter circuit, the transformer has nine three-phase secondary windings, in each case three of the nine three-phase secondary windings forming a set of secondary windings such that in total three sets of secondary windings are formed.
GB 2 330 254 A likewise discloses a polyphase converter circuit in which one converter circuit element is likewise provided for each phase, each converter circuit element having three rectifier units. A DC voltage circuit and an inverter unit which is connected to the DC voltage circuit are then connected to each rectifier unit. In addition, the converter circuit from GB 2 330 254 A has three transformers, each having a primary winding and three three-phase secondary windings. Furthermore, each secondary winding is associated with one of the three converter circuit elements, in this case each rectifier unit of the associated converter circuit element being connected to precisely one secondary winding associated with this converter circuit element.
One problem with a polyphase converter circuit in accordance with EP 0913 918 A2 is the fact that, despite the pulse number of 18 for the rectifier units and thus the converter circuit overall, harmonics which are relevant on the primary winding and thus in the electrical AC voltage system and which are greater than the seventeenth harmonic with respect to the fundamental of the voltage and the current of the electrical AC voltage system may occur, and these harmonics then place a correspondingly severe load on the electrical AC voltage system, in particular in the case of a weak electrical AC voltage system having a high rated impedance. Such effects of harmonics on the converter circuit are therefore highly undesirable.